This invention is directed to a method for treating granulated blast furnace slag for use in producing portland blast furnace slag cement and as a cementitious extender in the manufacture of concrete products.
Molten slag formed during the winning of iron from iron ores in a blast furnace is tapped from the blast furnace into slag pots and is transported to slag dumps for disposal. Because of the pollution and environmental problems associated with the disposal of the slag, several alternative uses of the blast furnace slag have been proposed. Slag with the proper composition has been fortified with certain chemicals and used as a fertilizer. Other slag because of its composition is processed with cementitious materials to produce a portland type cement, generally referred to as portland blast furnace slag cement. In the production of portland blast furnace slag cement, molten blast furnace slag is sprayed with streams of water to break up the molten slag into small glassy sand-like particles having a size consist of about 0.635 centimeter.times.0 mesh size (1/4 inch.times.0 mesh). The water/slag mixture thus formed contains about 85 to 95 weight percent water and 5 to 15 weight percent solids. The solids are separated from water prior to mixing with other materials used to manufacture portland blast furnace slag cement. Several processes in which the water is removed from the granulated slag particles are in use. In these processes, the slurry of the particles of granulated slag in water is passed to large vessels or settling tanks. The particles of slag settle to and are accumulated on the bottom of the vessels and the water passes out of the bottom of the vessels. A slurry containing the slag particles and a portion of water which remains in the vessels is accumulated in the vessels. The slurry contains as much as 17 weight percent water. The particles are stockpiled for a time to allow a portion of the water to drain naturally. It is virtually impossible to drain all the water during storage, therefore it is necessary to thermally remove the remaining moisture prior to adding the particles to the other materials required to produce cement. In the process it is necessary to use a plurality of large vessels to collect the particles of slag and partially dewater the slag. The necessity to store the slag particles for a period so that they may be partially dewatered and the subsequent thermal drying increase the cost of the portland blast furnace slag cement produced from the slag. Additionally, environmental control apparatus is required to prevent fine dust particles from passing into the atmosphere during thermal drying.
During storage the moist slag particles may harden because of chemical reactions between the materials in the slag. In the process of hardening, the cementitious materials, such as calcium sulfates, react with the glassy silicate portions of the slag in the presence of moisture and the resultant chemical, calcium silicate hydrate and/or calcium sulfoaluminate, forms a cementitious bond between the particles of slag. As a result, stored granules of slag frequently harden into a solid mass which cannot be easily handled. It is sometimes required to use dynamite to break up a hardened slag pile. A percentage of the stockpiled slag loses its cementitious properties and is then unsuitable for use in producing portland blast furnace slag cement.
There is, therefore, a need for a relatively simple, inexpensive method for treating granulated blast furnace slag for use in the production of portland blast furnace slag cement and as an extender to cementitious materials to produce concrete products, which method alleviates the aforementioned problems.
It is the object of this invention to provide a relatively simple, inexpensive method for treating granulated blast furnace slag for use as an addition to and extender for cementitious materials in which the slag is separated into two portions, one portion containing relatively coarse particles being suitable for use in the production of portland blast furnace slag cement and the other portion containing relatively fine particles being suitable for direct addition to cementitious materials which are used in the manufacture of concrete products.